Direct insertion of SiH₃ radicals into strained Si-Si surface bonds during plasma deposition of hydrogenated amorphous silicon films

We investigate the interaction of silyl (SiH₃) radicals with hydrogenated amorphous silicon (a-Si:H) films using real time in situ infrared spectroscopy in a mode that can detect as little as ∼0.2 ML of Si-H bonds. The results are directly relevant to the growth of a-Si:H by plasma-chemical vapor deposition. In this paper, a remote silane plasma source is used to generate a pure SiH₃ beam, without any contribution of H or SiH₂. Deuterated (a-Si:D) films are exposed to this beam and the change in the IR absorption caused by the loss of SiD groups and the creation of SiH groups is measured in real time. At the beginning of exposure to SiH₃ radicals, a hydrogen-rich layer is deposited on top of the deuterated sample, but no release of deuterium from SiD surface groups can be observed. This is a surprising result, since it has been assumed in the literature that SiH₃ radicals can easily abstract surface-bonded H atoms, and that abstraction must occur in order to create dangling bond sites on which SiH₃ radicals subsequently adsorb. Our results show that the reaction mechanism with the largest rate coefficient must be the direct insertion of SiH₃ radicals into bonding sites at the film surface, which leads to a hydrogen-rich top layer while preserving the preexisting SiD bonds. After completion of one monolayer of surface Si-H bonds, deuterium atoms from the initial surface are released simultaneously with the creation of SiH bulk groups. We propose a reaction scheme based on the direct insertion of SiH₃ radicals into strained Si-Si bonds. This scheme also predicts that the surface-bonding configurations depend on a dynamic balance between the rates of SiH₃ adsorption and thermal desorption, which is confirmed experimentally as a function of SiH₃ flux. We discuss the implications of this reaction mechanism for the growth of a-Si:H from silane discharges, and for the growth of microcrystalline Si in H₂ diluted SiH₄ discharges.